The ubiquitous tripeptide L-glutathione (GSH) (gamma-glutamyl-cysteinyl-glycine), is a well known biological antioxidant, and in fact is believed to be the primary intracellular antioxidant for higher organisms. When oxidized, it forms a dimer (GSSG), which may be recycled in organs having glutathione reductase. Glutathione may be transported through membranes by the sodium-dependent glutamate pump. Tanuguchi, N., et al. Eds., Glutathione Centennial, Academic Press, New York (1989), expressly incorporated herein by reference.
GSH is known to function directly or indirectly in many important biological phenomena, including the synthesis of proteins and DNA, transport, enzyme activity, metabolism, and protection of cells from free-radical mediated damage. GSH is one of the primary cellular antioxidants responsible for maintaining the proper oxidation state within the body. GSH is synthesized by most cells, and is also supplied in the diet. GSH has been shown to recycle oxidized biomolecules back to their active, reduced forms.
Reduced glutathione (GSH) is, in the human adult, produced from oxidized glutathione (GSSG) primarily by the liver, and to a smaller extent, by the skeletal muscle, red blood cells, and white cells. About 80% of the 8-10 grams glutathione produced daily is produced by the liver and distributed through the blood stream to the other tissues.
A deficiency of glutathione in cells may lead to excess free radicals, which cause macromolecular breakdown, lipid peroxidation, buildup of toxins, and ultimately cell death. Because of the importance of glutathione in preventing this cellular oxidation, glutathione is continuously supplied to the tissues. However, under certain conditions, the normal, physiologic supplies of glutathione are insufficient, distribution inadequate or local oxidative demands too high to prevent cellular oxidation. Under certain conditions, the production of and demand for glutathione are mismatched, leading to insufficient levels on an organismal level. In other cases, certain tissues or biological processes consume glutathione so that the intracellular levels are suppressed. In either case, by increasing the serum levels of glutathione, increased amounts may be directed into the cells. In facilitated transport systems for cellular uptake, the concentration gradient which drives uptake is increased.
As with all nutrients, it would normally be considered to eat or orally ingest the nutrient to increase body levels. Thus, attempts at oral glutathione treatments were known, and indeed the present inventors hereof previously suggested oral glutathione administration for various indications. The protocols for administration of glutathione, however, were not optimized and therefore the bioavailability of the glutathione was unassured and variable. All prior pharmaceutical attempts by others to safely, effectively and predictably raise intracellular GSH through oral therapy with GSH have not met with demonstrated success. Experts generally believe that beneficial physiological effects of orally administered glutathione are difficult or impossible to achieve, or the efficiency is so low as to make supplementation by this route unproductive.
Because of the poor or variable results obtained, the art generally teaches that oral administration of glutathione is ineffective, forcing administration or supplementation by other routes, principally intravenously, but also by alveolar inhalation. Orally absorbed prodrugs and precursors have also been proposed or used. A known pharmacological regimen provides intravenous glutathione in combination with another agent, such as cis-platinum (a free radical associated metal drug), doxorubicin, or daunorubicin (free radical associated drugs which interact with nucleic acid metabolism), which produced toxic side effects related to free radical reactions.
The ability to harness GSH, which is a powerful, but safe substance, into an effective oral pharmaceutical had not been accomplished in the past, because of molecular instability, poor gastrointestinal absorption through existing protocols and resulting inability to reliably effect increases in intracellular GSH levels. Administering sufficient amounts to achieve physiological benefit using known oral administration protocols might lead to cysteine related kidney stones, gastric distress or flatulence.
Glutathione is relatively unstable in alkaline or oxidative environments, and is not absorbed by the stomach. It is believed that glutathione is absorbed, after oral administration, if at all, in the latter half of the duodenum and the beginning of the jejunum. It was also believed that orally administered glutathione would tend to be degraded in the stomach, and that it is particularly degraded under alkaline conditions by desulfurases and peptidases present in the duodenum. Thus, known protocols for oral administration of glutathione involved administered with meals or after eating to buffer pH extremes and dilute degradative enzymes. This protocol, however, has the effect of diluting the glutathione and delaying absorption. Studies directed at determining the oral bioavailability of glutathione under such circumstances showed poor absorption, and therefore such administration was seen as of little benefit.
Therefore, while oral dosage forms of glutathione were known, the clinical benefits of these formulations were unproved and, given the lack of predictability of their effect, these formulations were not used for the treatment of specific conditions, nor proven to have effect. Further, the known protocols for administration of glutathione did not provide convenience and high bioavailability.
The prior art thus suggests that glutathione esters might be suitable as orally bioavailable sources of glutathione, which are stable and may be rapidly absorbed. However, these are both more expensive than glutathione itself and have proven toxic.
Pure glutathione forms a flaky powder which retains a static electrical charge, due to triboelectric effects, that makes processing difficult. The powder may also have an electrostatic polarization, which is akin to an electret. Glutathione is a strong reducing agent, so that autooxidation occurs in the presence of oxygen or other oxidizing agents. U.S. Pat. No. 5,204,114, Demopoulos et al., expressly incorporated herein by reference in its entirety, provides a method of manufacturing glutathione tablets and capsules by the use of crystalline ascorbic acid as an additive to reduce triboelectric effects which interfere with high speed equipment and maintaining glutathione in a reduced state. A certain crystalline ascorbic acid is, in turn, disclosed in U.S. Pat. No. 4,454,125, Demopoulos, expressly incorporated by reference herein in its entirety. This crystalline form is useful as a lubricating agent for machinery. Ascorbic acid has the advantage that it is well tolerated, antioxidant, and reduces the net static charge on the glutathione.
In synthesizing glutathione in the body, cysteine, a thiol amino acid is required. Since the prior art suggests that oral administration of glutathione itself would be ineffective, prodrugs or precursor therapy was advocated. Therefore, the prior art suggests administration of cysteine, or a more bioavailable precursor of cysteine, N-acetyl cysteine (NAC). While cysteine and NAC are both, themselves, antioxidants, their presence competes with glutathione for resources in certain reducing (GSH recycling) pathways. Since glutathione is a specific substrate for many reducing pathways, the loading of a host with cysteine or NAC may result in less efficient utilization or recycling of glutathione. Thus, cysteine and NAC are not ideal GSH prodrugs. Thus, while GSH may be degraded, transported as amino acids, and resynthesized in the cell, there may also be circumstances where GSH is transported into cells without degradation; and in fact the administration of cysteine or cysteine precursors may interfere with this process.
A number of disease states have been specifically associated with reductions in glutathione levels. Depressed glutathione levels, either locally in particular organs, or systemically, have been associated with a number of clinically defined diseases and disease states. These include HIV/AIDS, diabetes and macular degeneration, all of which progress because of excessive free radical reactions and insufficient GSH. Other chronic conditions may also be associated with GSH deficiency, including heart failure and coronary artery restenosis post angioplasty.
For example, diabetes afflicts 8% of the United States population and consumes nearly 15% of all United States healthcare costs. HIV/AIDS has infected nearly 1 million Americans. Current therapies cost in excess of $20,000 per year per patient, and are rejected by, or fail in 25% to 40% of all patients. Macular degeneration presently is considered incurable, and will afflict 15 million Americans by 2002.
Clinical and pre-clinical studies have demonstrated the linkage between a range of free radical disorders and insufficient GSH levels. Newly published data implies that diabetic complications are the result of hyperglycemic episodes that promote glycation of cellular enzymes and thereby inactivate GSH synthetic pathways. The result is GSH deficiency in diabetics, which may explain the prevalence of cataracts, hypertension, occlusive atherosclerosis, and susceptibility to infections in these patients.
GSH functions as a detoxicant by forming GSH S-conjugates with carcinogenic electrophilcs, preventing reaction with DNA, and chelation complexes with heavy metals such as nickel, lead, cadmium, mercury, vanadium, and manganese. GSH also plays a role in metabolism of various drugs, such as opiates. It has been used as an adjunct therapy to treatment with nephrotoxic chemotherapeutic agents such as cisplatin, and has been reported to prevent doxorubicin-induced cardiomyopathy. GSH is also an important factor in the detoxification of acetaminophen and ethanol, two powerful hepatotoxins.
(I) HIV
High GSH levels have been demonstrated to be necessary for proper functioning of platelets, vascular endothelial cells, macrophages, cytotoxic T-lymphocytes, and other immune system components. Recently it has been discovered that HIV-infected patients exhibit low GSH levels in plasma, in other fluids, and in certain cell types like macrophages, which does not appear to be due to defects in GSH synthesis. GSH has been shown to inhibit HIV replication in chronically-infected cells and in cells acutely infected in vitro. This makes GSH replacement therapy attractive, because it has the potential to interfere with the expression of the integrated HIV genome, a site that is not attacked by the currently employed antiretrovirals (AZT, ddI, ddC, D4T). GSH may also have benefits in countering the excess free radical reactions in HIV infection, which may be attributable to: 1) the hypersecretion of TNF-α by B-lymphocytes, in HIV infection, and 2) the catalysis of arachidonic acid metabolism by the gp 120 protein of HIV. The physiologic requirements for GSH by key cell types of the immune system, and the ability of macrophages to take up intercellular GSH, as well as to metabolically interact with T-lymphocytes to indirectly cause their GSH to increase, offer additional reasons to attempt to correct the GSH deficiency in HIV/AIDS.
In other new data dealing with HIV infections, the March 1997 issue of the Proceedings of the National Academy of Sciences (PNAS) established “. . . GSH deficiency as a key determinant of survival in HIV disease . . . ” GSH deficiency is associated with impaired survival in HIV disease (PNAS. Vol. 94, pp. 1967-1972). The quest to raise GSH levels in cells is widely recognized as being extremely important in HIV/AIDS and other disorders, because the low cellular GSH levels in these disease processes permit more and more free radical reactions to propel the disorders.
HIV is known to start pathologic free radical reactions which lead to the destruction of GSH, as well as exhaustion of other antioxidant systems and destruction of cellular organelles and macromolecules. In pre-clinical studies, GSH stops the replication of the virus at a unique point, and specifically prevents the production of toxic free radicals, prostaglandins, TNF-α, interleukins, and a spectrum of oxidized lipids and proteins that are immunosuppressive, cause muscle wasting and neurologic symptoms. Restoring GSH levels could slow or stop the diseases progression, safely and economically.
In mammalian cells, oxidative stresses, i.e., low intracellular levels of reduced GSH, and relatively high levels of free radicals, activate certain cytokines, including NFκB and TNF-α, which, in turn, activate cellular transcription of the DNA to mRNA, resulting in translation of the mRNA to a polypeptide sequence. In a virus infected cell, the viral genome is transcribed, resulting in viral RNA production, generally necessary for viral replication of RNA viruses and retroviruses. These processes require a relatively oxidized state of the cell, a condition which results from stress, low glutathione levels, or the production of reduced cellular products. The mechanism which activates cellular transcription is evolutionarily highly conserved, and therefore it is unlikely that a set of mutations would escape this process, or that an organism in which mutated enzyme and receptor gene products in this pathway would be well adapted for survival. Thus, by maintaining a relatively reduced state of the cell (redox potential), viral transcription, a necessary step in late stage viral replication, is impeded.
The amplification effect of oxidative intracellular conditions on viral replication is compounded by the actions of various viruses and viral products which degrade GSH. For example, GP-120, an HIV surface glycoprotein having a large number of disulfide bonds, and normally present on the surface of infected cells, oxidizes GSH, resulting in reduced intracellular GSH levels. On the other hand, GSH reduces disulfide bonds of GP-120, reducing or eliminating its biological activity, necessary for viral infectivity. GSH therefore interferes with the production of such oxidized proteins, and degrades them once formed. In a cell which is actively replicating viral gene products, a cascade of events may occur which allow the cell to pass from a relatively quiescent stage with low viral activity to an active stage with massive viral replication and cell death, accompanied by a change in redox potential; by maintaining adequate GSH levels, this cascade may be impeded.
Thus, certain viral infections, such as HIV, are associated with reduced GSH levels, and it is believed that by increasing intracellular GSH levels in infected cells, as well as increasing extracellular GSH, the replication of HIV may be interfered with, and the cascade of events delayed or halted. It is noted that AIDS may also be associated with reduced GSSG levels, implying an interference with de novo synthesis of GSH as well as the oxidation of existing GSH discussed above.
The Human Immunodeficiency Virus (HIV) is transmitted through two predominant routes, contaminated blood and/or sexual intercourse. In pediatric cases, approximately one half are infected in utero, and one half at delivery. This circumstance permits a study of prevention of transmission since the time of spread is known. Initially, there is an intense viral infection simulating a severe case of the flu, with massive replication of the virus. This acute phase passes within weeks, spontaneously, as the body mounts a largely successful immune defense. Thereafter, the individual has no outward manifestations of the infection. However, the virus continues to replicate, insidiously, within immune system tissues and cells, like lymph nodes, lymphoid nodules and special multidendritic cells that are found in various body cavities.
This infection is not just a viral problem. The virus, in addition to replicating, causes excessive production of various free radicals and various cytokines in toxic or elevated levels. The latter are normally occurring biochemical substances that signal numerous reactions, usually exist in minuscule concentrations. Eventually, after an average of 7-10 years, of seemingly quiescent HIV infection, the corrosive free radicals and the toxic levels of cytokines begin to cause symptoms, and failures in the immune system begin. Substances like 15-HPETE are immunosuppressive and TNF-α causes muscle wasting, among other toxic factors. The numbers of viral particles increase and the patient develops the Acquired Immune Deficiency Syndrome, AIDS, which may last 2 to 4 years before the individual's demise. AIDS, therefore, is not simply a virus infection, although the viral infection is believed to be an integral part of the etiology of the disease.
HIV has a powerful ability to mutate. It is this capability that makes it difficult to create a vaccine or to develop long-term anti-viral pharmaceutical treatments. As more people continue to fail the present complex regimens, the number of resistant viral strains is increasing. This is a particularly dangerous pool of HIV and poses a considerable threat. These resistant mutants also add to the difficulties in developing vaccines. This epidemic infection is out of control, and the widely popularized polypharmaceutical regimens that are aimed only at lowering the number of viruses are proving to be too complex, too toxic, too costly, and too narrow. As a result, in the past 1.5 years since the introduction of protease inhibitors, in combination with AZT-type drugs, increasing numbers of people are failing therapy, approximately 25% and growing. Further, the continuing production of free radicals and cytokines that may become largely independent of the virus, perpetuate the dysfunctions of the immune system, the gastrointestinal tract, the nervous system, and many other organs in AIDS. The published scientific literature indicates that many of these diverse organ system dysfunctions are due to systemic GSH deficiencies that are engendered by the virus and its free radicals. GSH is consumed in HIV infections because it is the principal, bulwark antioxidant versus free radicals. An additional cause of erosion of GSH levels is the presence of numerous disulfide bonds (—S—S—) in HIV proteins, such as the GP-120 discussed above. Disulfide bonds react with GSH and oxidize it.
This disease obviously is not controllable with the present approaches and basically can not be curtailed in its spread merely by superficial public health messages regarding safe sex and clean needles, or by using overly complex, toxic, costly medications that are narrowly aimed at just viral replication.
The current HIV/AIDS pharmaceuticals take good advantage of the concept of pharmaceutical synergism, wherein two different targets in one process are hit simultaneously. The effect is more than additive. The drugs now in use were selected to inhibit two very different points in the long path of viral replication. The pathway of viral replication can be depicted simply:
HIV Replication Pathway−−−−−−→−−−−−−→−−−−−−→−−−−−−→−−−−−−→point #1point #2point #3point #4point #5Virus attacksVirus makesViral DNA isProviral DNAViral RNA isand enters theDNA from itsintegrated intois inactive for aproduced, alongcellRNAcells' DNAlong time, butwith viralactivators willmembranes andstart HIVproteins, whichreplicatingare assembledrapidlyViral gp120ReverseIntegrase is theNF kappa B isViral proteaseprotein andtranscriptase isenzymethe activator ofis involvedCD4+ cellthe enzymeinvolveddormant HIVreceptors andinvolvedDNA andothers areglutathioneinvolvedlevels must below foractivation tooccurAZT, ddl, ddCGlutathioneProteaseInhibitors
Point #2 was the earliest point of attack, using AZT-types of drugs, including ddI, ddC and others. These are toxic and eventually viruses become resistant to these Reverse Transcriptase inhibitors.
Point #5 is a late replication step, and this is where protease inhibitors function. The drug blocks viral protease, an enzyme that snips long protein chains to just the right length so the viral coat fits exactly around the nucleic acid core, and that proteins having different biological activities are separated. By themselves, protease inhibitors foster the rapid development of resistant, mutant strains.
By combining Reverse Transcriptase inhibitors plus protease inhibitors, synergism was obtained and the amounts of viral particles in the plasma plummeted, while the speed of the developing mutant resistant viral strains was slowed, compared to using only one type of inhibitor. This combination has been in use for about 1.5 years, and so far, about 25% to 40% of U.S. patients have failed the treatment. This number is expected to rise as resistant mutants develop, albeit more slowly than the use of the drugs separately.
In addition to the multiple drugs aimed at the virus, at points #2 and #5, AIDS patients and progressing HIV positive people who have not yet developed an AIDS-related disease, also take other pharmaceuticals, the most common being one to prevent the unusual pneumonia caused by Pneumocystis carinii, for example trimethoprim-sulfathiazole. As other opportunistic infections occur with fungi, yeasts, bacteria, tuberculosis, and other viruses like cytomegalovirus infection of the retinae, the number of pharmaceuticals increases greatly. Sadly, AIDS patients are also more likely to develop cancers, such as lymphomas, cancer of the cervix and Kaposi's sarcoma. Management of the cancers requires the addition of still more drugs.
New therapies include additional drugs in the classes of Reverse Transcriptase inhibitors and protease inhibitors. Also, drugs are in development to block point #3, wherein the enzyme, integrase, integrates the HIV DNA into the infected cell's DNA, analogous to splicing a small length of wire into a longer wire. Vaccine development also continues, although prospects seem poor because HIV appears to be a moving target and seems to change as rapidly as a chameleon. Vaccine development is also impaired by the immune cell affinity of the virus.
Human Immunodeficiency virus-infected individuals have lowered levels of serum acid-soluble thiols and GSH in plasma, peripheral blood monocytes, and lung epithelial lining fluid. In addition, it has been shown that CD4+ and CD8+ T cells with high intracellular GSH levels are selectively lost as HIV infection progresses. This deficiency may potentiate HIV replication and accelerate disease progression, especially in individuals with increased concentrations of inflammatory cytokines because such cytokines stimulate HIV replication more efficiently in GSH-depleted cells. GSH and glutathione precursors such as N-acetyl cysteine (NAC) can inhibit cytokine-stimulated HIV expression and replication in acutely infected cells, chronically infected cells, and in normal peripheral blood mononuclear cells.
It is noted that depletion of GSH is also associated with a processes known as apoptosis, or programmed cell death. Thus, intercellular processes which artificially deplete GSH may lead to cell death, even if the process itself is not lethal.
2) Diabetes Mellitus
Diabetes mellitus is found in two forms, childhood or autoimmune (type I, IDDM) and late-onset or non-insulin dependent (type II, NIDDM). The former constitute about 30% and the remainder represent the bulk of cases seen. Onset is generally sudden for Type I, and insidious for Type II. Symptoms include excessive urination, hunger and thirst with a slow steady loss of weight in the first form. Obesity is often associated with the second form and has been thought to be a causal factor in susceptible individuals. Blood sugar is often high and there is frequent spilling of sugar in the urine. If the condition goes untreated, the victim may develop ketoacidosis with a foul-smelling breath similar to someone who has been drinking alcohol. The immediate medical complications of untreated diabetes can include nervous system symptoms, and even diabetic coma.
Because of the continuous and pernicious occurrence of hyperglucosemia (very high blood sugar levels), a non-enzymatic chemical reaction occurs called glycation. Since glycation occurs far more frequently inside cells, the inactivation of essential enzyme proteins happens almost continually. One of the most critical enzymes, γ-glutamyl-cysteine synthetase, is glycated and readily inactivated. This enzyme is the crucial step in the biosynthesis of glutathione in the liver.
The net result of this particular glycation is a deficiency in the production of GSH in diabetics. Normally, adults produce 8-10 grams every 24 hours, and it is rapidly oxidized by the cells. GSH is in high demand throughout the body for multiple, essential functions, for example, within all mitochondria, to produce chemical energy called ATP. Brain cells, heart cells, and others simply will not function well and can be destroyed through apoptosis.
GSH is the major antioxidant in the human body and the only one we are able to synthesize, de novo. It is also the most common small molecular weight thiol in both plants and animals. Without GSH the immune system cannot function, and the central and peripheral nervous systems become aberrant and then cease to function. Because of the dependence on GSH as the carrier of nitric oxide, a vasodilator responsible for control of vascular tone, the cardiovascular system does not function well and eventually fails. Since all epithelial cells seem to require GSH, the intestinal lining cells don't function properly and valuable micronutrients are lost, nutrition is compromised, and microbes are given portals of entry to cause infections.
The use of GSH precursors cannot help to control the GSH deficiency due to the destruction of the rate-limiting enzyme by glycation. As GSH deficiency becomes more profound, the well-known sequellae of diabetes progress in severity. The complications described below are essentially due to runaway free radical damage since the available GSH supplies in diabetics are insufficient.
The diabetic will become more susceptible to infections because the immune system approaches collapse when GSH levels fall . . . analogous to HIV/AIDS. Peripheral vasculature becomes compromised and blood supply to the extremities is severely diminished because GSH is not available in sufficient amounts to stabilize the nitric oxide (•NO) to effectively exert its vascular dilation (relaxation) property. Gangrene is a common sequel and successive amputations are often the result in later years.
Peripheral neuropathies, the loss of sensation commonly of the feet and lower extremities develop, often followed by aberrant sensations like burning or itching which can't be controlled. Retinopathy and nephropathy are later events which are actually due to microangiopathy, excessive budding and growth of new blood vessels and capillaries, which often will bleed due to weakness of the new vessel walls. This bleeding causes damage to the retina and kidneys with resulting blindness and renal shutdown, the latter results in required dialysis. Cataracts occur with increasing frequency as the GSH deficiency deepens.
Large and medium sized arteries become sites of accelerated, severe atherosclerosis, with myocardial infarcts at early ages, and of a more severe degree. If diabetics go into heart failure, their mortality rates at one year later are far greater than in non-diabetics. Further, if coronary angioplasty is used to treat their severe atherosclerosis, diabetics are much more likely to have renarrowing of cardiac vessels, termed restenosis.
The above complications are due, in large measure, to GSH deficiency and ongoing free radical reactions. These sequellae frequently and eventually occur despite the use of insulin injections daily that lower blood sugar levels. Good control of blood sugar levels is difficult for the majority of diabetics.
3) Macular Degeneration
Approximately 1 million people in the United States have significant macular degeneration. One out of every 4 persons aged 55 or above now has macular degeneration and 1 in 2 above the age of 80. As our population ages this principal cause of blindness in the elderly will increase as well. By the year 2002, 15 million people in the U.S. will suffer from macular degeneration.
Age-related macular degeneration (ARMD) is the disease characterized by either a slow (dry form) or rapid (wet form) onset of destruction and irrevocable loss of rods and cones in the macula of the eye. The macula is the approximate center of the retina wherein the lens of the eye focuses its most intense light. The visual cells, known as the rods and cones, are an outgrowth and active part of the central nervous system. They are responsible and essential for the fine visual discrimination required to see clear details such as faces and facial expression, reading, driving, operation of machinery and electrical equipment and general recognition of surroundings. Ultimately, the destruction of the rods and cones leads to functional, legal blindness. Since there is no overt pain associated with the condition, the first warnings of onset are usually noticeable loss of visual acuity. This may already signal late stage events. It is now thought that one of the very first events in this pathologic process is the formation of a material called “drusen”.
Drusen first appears as either patches or diffuse drops of yellow material deposited upon the surface of the retina in the macula lutea or yellow spot. This is the area of the retina where sunlight is focused by the lens. It is the area of the retina which contains the highest density of rods for acuity. Although cones, which detect color are lost as well in this disease, it is believed to be loss of rods which causes the blindness. Drusen has been chemically analyzed and found to be composed of a mixture of lipids much of it peroxidized by free radical reactions. The Drusen first appears as small collections of material at the base of Bruch's membrane. This produces “bubbles” which push the first layer of cells up off the membrane. Vascular budding, neovascular growth, first appears in these channels. This first layer of cells is unique.
They are retinal pigmented epithelial (RPE) cells and these cells are distantly related to CNS microglia and have a phagocytic function. They are also the layer of cells immediately below the primary retinal cells, the rods and cones. The RPE cells are believed to serve a protective function for the rods and cones since they consume the debris cast off by the rods and cones. It is not known yet whether the pigmented material serves a protective function or is related to phagocytosis only. However, this pigment although concentrated in organelles, is believed to be composed of peroxidized lipids and melanin.
It is believed, because of the order of events in model systems, that the loss of RPE cells occurs first in ARMD (Age Related Macular Degeneration). Once an area of the retinal macula is devoid of RPE cells, loss of rods, and eventually some cones, occurs. Finally, budding of capillaries begins and we see the typical microangiopathy associated with late stage ARMD. It is also known that RPE cells require large quantities of GSH for their proper functioning. When GSH levels drop severely in these cells, in cell cultures where they can be studied, these cells begin to die. When cultures of these cells are supplemented with GSH in the medium, they thrive. There is increasing evidence that progression of the disease is paced by a more profound deficiency in GSH within the retina and probably within these cells, as indicated by cell culture studies.
It is generally believed that “near” ultraviolet (UVB) and visual light of high intensity primarily from sunlight is a strong contributing factor of ARMD. People with light-colored irises constitute a population at high risk, as do those with jobs which leave them outdoors and in equatorial areas where sunlight is most intense. Additional free radical insults, like smoking, adds to the risk of developing ARMD.
Several approaches have been recently tested, including chemotherapy, without success. Currently, there is no effective therapy to treat ARMD. Laser therapy has been developed which has been used widely to slow the damage produced in the slow onset form of the disease by cauterizing neovascular growth. However the eventual outcome of the disease, once it has started to progress, is certain.
Metabolism of Glutathione
The synthesis of GSH is dependent upon the availability of cysteine either supplied directly from the diet or cysteine or indirectly from methionine via the transsulfuration pathway. GSH synthesis and metabolism is governed by the enzymes of the γ-glutamyl cycle as shown in FIG. 1. GSH is synthesized intracellularly by the consecutive actions of γ-glutamylcysteinyl synthetase (Reaction 1) and GSH synthetase (Reaction 2). The action of the latter enzyme is feedback inhibited by GSH. The breakdown of GSH (and also of its oxidized form, GSSG) is catalyzed by γ-glutamyl transpeptidase, which catalyzes the transfer of the gamma-glutamyl moiety to acceptors such as sulfhydryl-containing amino acids, certain dipeptides, and GSH itself (Reaction 3). The cellular turnover of GSH is associated with its transport, in the form of GSH, across cell membranes, where the majority of the transpeptidase is found. During this transport, GSH interacts with γ-glutamyl transferase (also known as transpeptidase) to form γ-glutamyl amino acids which are transported into cells. Intracellular γ-glutamyl amino acids are substrates of γ-glutamyl cyclotransferase (Reaction 4) which converts these compounds into the corresponding amino acids and 5-oxo-L-proline. The ATP-dependent conversion of 5-L-oxoproline to L-glutamate is catalyzed by the intracellular enzyme 5-oxo-prolinase (Reaction 5). The cysteinylglycine formed in the transpeptidase reaction is split by dipeptidase (Reaction 6). These six reactions constitute the γ-glutamyl cycle, which accounts for the synthesis and enzymatic degradation of GSH.
Two of the enzymes of the cycle also function in the metabolism of S-substituted GSH derivatives, which may be formed nonenzymatically by reaction of GSH with certain electrophilic compounds or by GSH S-transferases (Reaction 7). The γ-glutamyl moiety of such conjugates is removed by the action of γ-glutamyl transpeptidase (Reaction 3), a reaction facilitated by γ-glutamyl amino acid formation. The resulting S-substituted cysteinylglycines are cleaved by dipeptidase (Reaction 6A) to yield the corresponding S-substituted cysteines, which may undergo N-acetylation (Reaction 8) or an additional transpeptidation reaction to form the corresponding γ-glutamyl derivative (Reaction 3A).
Intracellular GSH is converted to its oxidized, dimeric form (GSSG) by selenium-containing GSH peroxidase, which catalyzes the reduction of H2O2 and other peroxides (Reaction 9). GSH is also converted to GSSG by transhydrogenation (Reaction 10). Reduction of GSSG to GSH is mediated by the widely-distributed enzyme GSSG reductase which uses NADPH (Reaction 11). Extracellular conversion of GSH to GSSG has also been reported; the overall reaction requires O2 and leads to the formation of H2O2 (Reaction 12). GSSG is also formed by reaction of GSH with free radicals.
Transport of Glutathione
The intracellular level of GSH in mammalian cells is in the range of 0.5-10 millimolar, while micromolar concentrations are typically found in blood plasma. Intracellular glutathione is normally over 99% reduced form (GSH). The normal healthy adult human liver synthesizes 8-10 grams of GSH daily. Normally, there is an appreciable flow of GSH from liver into plasma. The major organs involved in the inter-organ transport of GSH are the liver and the kidney, which is the primary organ for clearance of circulating GSH. It has been estimated to account for 50-67% of net plasma GSH turnover. Several investigators have found that during a single pass through the kidney, 80% or more of the plasma GSH is extracted, greatly exceeding the amount which could be accounted for by glomerular filtration. While the filtered GSH is degraded stepwise by the action of the brush-border enzymes γ-glutamyltransferase and cysteinyglycine dipeptidase, the remainder of the GSH appears to be transported via an unrelated, Na+-dependent system present in basal-lateral membranes.
GSH transported from hepatocytes interacts with the transpeptidase of ductile cells, and there appears to be a substantial reabsorption of metabolites by ductule endothelium. In the rat, about 12 and 4 nmoles/g/min of GSH appear in the hepatic vein and bile, respectively. Glutathione exists in plasma in four forms: reduced glutathione (GSH), oxidized glutathione (GSSG), mixed disulfide with cysteine (CySSG) and protein bound through a sulfhydryl linkage (GSSPr). The distribution of glutathione equivalents is significantly different than that of cyst(e)ine, and when either GSH or cysteine is added at physiological concentration, a rapid redistribution occurs. The distribution of glutathione equivalents in rat plasma is 70.0% protein bound, with the remaining 30% apportioned as follows: 28.0% GSH, 9.5% GSSG, and 62.6% as the mixed disulfide with cysteine. The distribution of cysteine equivalents was found to be 23% protein bound, with the remaining 77% distributed as follows: 5.9% cysteine, 83.1% cystine, and 10.8% as the mixed disulfide with glutathione. Plasma thiols and disulfides are not in equilibrium, but appear to be in a steady state maintained in part by transport of these compounds between tissues during the interorgan phase of their metabolism. The large amounts of protein-bound glutathione and cysteine provide substantial buffering which must be considered in the analysis of transient changes in glutathione and cysteine. This buffering may protect against transient thiol-disulfide redox changes which could affect the structure and activity of plasma and plasma membrane proteins. In erythrocytes, GSH has been implicated in reactions which maintain the native structure of hemoglobin and of enzymes and membrane proteins. GSH is represent in erythrocytes at levels 1000 times greater than in plasma. It functions as the major small molecule antioxidant defense against toxic free radicals, an inevitable by-product of the erythrocytes' handling of O2.
Glutathione and the Immune System
The importance of thiols and especially of GSH to lymphocyte function has been known or many years. Adequate concentrations of GSH are required for mixed lymphocyte reactions, T-cell proliferation, T- and B-cell differentiation, cytotoxic T-cell activity, and natural killer cell activity. Adequate GSH levels have been shown to be necessary for microtubule polymerization in neutrophils. Intraperitoneally administered GSH augments the activation of cytotoxic T-lymphocytes in mice, and dietary GSH was found to improve the splenic status of GSH in aging mice, and to enhance T-cell-mediated immune responses.
The presence of macrophages can cause a substantial increase of the intracellular GSH levels of activated lymphocytes in their vicinity. Macrophages consume cystine via a strong membrane transport system, and generate large amounts of cysteine which they release into the extracellular space. It has been demonstrated that macrophage GSH levels (and therefore cysteine equivalents) can be augmented by exogenous GSH. T-cells cannot produce their own cysteine, and it is required by T-cells as the rate-limiting precursor of GSH synthesis. The intracellular GSH level and the DNA synthesis activity in mitogenically-stimulated lymphocytes are strongly increased by exogenous cysteine, but not cystine. In T-cells, the membrane transport activity for cystine is ten-fold lower than that for cysteine. As a consequence, T-cells have a low baseline supply of cysteine, even under healthy physiological conditions. The cysteine supply function of the macrophages is an important part of the mechanism which enables T-cells to shift from a GSH-poor to a GSH-rich state.
The importance of the intracellular GSH concentration for the activation of T-cells is well established. It has been reported that GSH levels in T-cells rise after treatment with GSH; it is unclear whether this increase is due to uptake of the intact GSH or via extracellular breakdown, transport of breakdown products, and subsequent intracellular GSH synthesis. Decreasing GSH by 10-40% can completely inhibit T-cell activation in vitro. Depletion of intracellular GSH has been shown to inhibit the mitogenically-induced nuclear size transformation in the early phase of the response. Cysteine and GSH depletion also affects the function of activated T-cells, such as cycling T-cell clones and activated cytotoxic T-lymphocyte precursor cells in the late phase of the allogenic mixed lymphocyte culture. DNA synthesis and protein synthesis in IL-2 dependent T-cell clones, as well as the continued growth of preactivated CTL precursor cells and/or their functional differentiation into cytotoxic effector cells are strongly sensitive to GSH depletion.
The activation of physiologic activity of mouse cytotoxic T-lymphocytes in vivo was found to be augmented by interperitoneal (i.p.) GSH in the late phase but not in the early phase of the response. The injection of GSH on the third day post immunization mediated a 5-fold augmentation of cytotoxic activity. Dietary GSH supplementation can reverse age-associated decline of immune response in rats, as demonstrated by maintenance of Concanavalin A stimulated proliferation of splenocytes in older rats.
Glutathione status is a major determinant of protection against oxidative injury. GSH acts on the one hand by reducing hydrogen peroxide and organic hydroperoxides in reactions catalyzed by glutathione peroxidases, and on the other hand by conjugating with electrophilic xenobiotic intermediates capable of inducing oxidant stress. The epithelial cells of the renal tubule have a high concentration of GSH, no doubt because the kidneys function in toxin and waste elimination, and the epithelium of the renal tubule is exposed to a variety of toxic compounds. GSH, transported into cells from the extracellular medium, substantially protects isolated cells from intestine and lung are against t-butylhydroperoxide, menadione or paraquat-induced toxicity. Isolated kidney cells also transport GSH, which can supplement endogenous synthesis of GSH to protect against oxidant injury. Hepatic GSH content has also been reported to rise, indeed to double, in the presence of exogenous GSH. This may be due either to direct transport, as has been reported for intestinal and alveolar cells, or via extracellular degradation, transport, and intracellular resynthesis.
The nucleophilic sulfur atom of the cysteine moiety of GSH serves as a mechanism to protect cells from harmful effects induced by toxic electrophiles. The concept that glutathione S-conjugate biosynthesis is an important mechanism of drug and chemical detoxification is well established. GSH conjugation of a substrate generally requires both GSH and glutathione-S-transferase activity. The existence of multiple glutathione-S-transferases with specific, but also overlapping, substrate specificities enables the enzyme system to handle a wide range of compounds.
Several classes of compounds are believed to be converted by glutathione conjugate formation to toxic metabolites. Halogenated alkenes, hydroquinones, and quinones have been shown to form toxic metabolites via the formation of S-conjugates with GSH. The kidney is the main target organ for compounds metabolized by this pathway. Selective toxicity to the kidney is the result of the kidney's ability to accumulate intermediates formed by the processing of S-conjugates in the proximal tubular cells, and to bioactivate these intermediates to toxic metabolites.
The administration of morphine and related compounds to rats and mice results in a loss of up to approximately 50% of hepatic GSH. Morphine is known to be biotransformed into morphinone, a highly hepatotoxic compound, which is 9 times more toxic than morphine in mouse by subcutaneous injection, by morphine 6-dehydrogenase activity. Morphinone possesses an α,β-unsaturated ketone, which allows it to form a glutathione S-conjugate. The formation of this conjugate correlates with loss of cellular GSH. This pathway represents the main detoxification process for morphine. Pretreatment with GSH protects against morphine-induced lethality in the mouse.
The deleterious effects of methylmercury on mouse neuroblastoma cells are largely prevented by coadministration of GSH. GSH may complex with methylmercury, prevent its transport into the cell, and increase cellular antioxidant capabilities to prevent cell damage. Methylmercury is believed to exert its deleterious effects on cellular microtubules via oxidation of tubulin sulfhydryls, and by alterations due to peroxidative injury. GSH also protects against poisoning by other heavy metals such as nickel and cadmium.
Because of its known role in renal detoxification and its low toxicity, GSH has been explored as an adjunct therapy for patients undergoing cancer chemotherapy with nephrotoxic agents such as cisplatin, in order to reduce systemic toxicity. In one study, GSH was administered intravenously to patients with advanced neoplastic disease, in two divided doses of 2,500 mg, shortly before and after doses of cyclophosphlamide. GSH was well-tolerated and did not produce unexpected toxicity. The lack of bladder damage, including microscopic hematuria, supports the protective role of this compound. Other studies have shown that i.v. GSH coadministration with cisplatin and/or cyclophosphamide combination therapy, reduces associated nephrotoxicity, while not unduly interfering with the desired cytotoxic effect of these drugs.
Clinical Use of Glutathione
Ten elderly patients with normal glucose tolerance and ten elderly patients with impaired glucose tolerance (IGT) underwent GSH infusion, 10 mg/min for 120 min, for a total dose of 1,200 mg in 2 hr, under basal conditions and during 75 g oral glucose tolerance tests and intravenous glucose tolerance tests. Basal plasma total glutathione levels were essentially the same for normal and IGT groups, and GSH infusion under basal conditions increased GSH to similar levels. This study demonstrated that GSH significantly potentiated glucose-induced insulin secretion in patients with IGT. No effect was seen on insulin clearance and action.
The antihypertensive effect of an i.v. bolus of 1,844 mg. or 3,688 mg. GSH was studied in normal and mild to moderate essential hypertensive subjects and in both hypertensive and non-hypertensive diabetics, both type I and type II. The administration of 1,844 mg. GSH produced a rapid and significant decrease in both systolic and diastolic blood pressure, within ten minutes, but which returned to baseline within 30 minutes, in both groups of hypertensive patients and in non-hypertensive diabetics, but had no effect in normal healthy subjects. At the 3,699 mg. dose, not only did the blood pressure decrease in the hypertensive subjects, but GSH produced a significant decrease in the blood pressure values in normal subjects as well.
GSH, 1,200 mg/day intravenously administered to chronic renal failure patients on hemodialysis was found to significantly increase studied hematologic parameters (hematocrit, hemoglobin, blood count) as compared to baseline, and holds promise to reverse the anemia seen in these patients.
Toxicological Effects of Glutathione
The reported LD50 of GSH in rats and mice via various routes of administration are listed in the table below. GSH has an extremely low toxicity, and oral LD50 measurements are difficult to perform due to the sheer mass of GSH which has to be ingested by the animal in order to see any toxic effects.
Route ofAnimalAdmin.LD50ReferenceMouseOral5000 mg/kgModern Pharmaceuticals of Japan, IV Edition,Tokyo, Japan Pharmaceutical, Medical andDental Supply Exporters' Association, 1972, p93.MouseIntraperitoneal4020 mg/kgModern Pharmaceuticals of Japan, IV Edition,Tokyo, Japan Pharmaceutical, Medical andDental Supply Exporters' Association, 1972, p93.MouseIntraperitoneal6815 mg/kgToxicology, vol. 62, P. 205, 1990.MouseSubcutaneous5000 mg/kgModern Pharmaceuticals of Japan, IV Edition,Tokyo, Japan Pharmaceutical, Medical andDental Supply Exporters' Association, 1972, p93.MouseIntravenous2238 mg/kgJapanese J. of Antibiotics, vol. 38, p. 137,1985.MouseIntramuscular4000 mg/kgModern Pharmaceuticals of Japan, III Edition,Tokyo, Japan Pharmaceutical, Medical andDental Supply Exporters' Association, 1968, p97.
GSH can be toxic, especially in cases of ascorbate deficiency, and these effects may be demonstrated in, for example, ascorbate deficient guinea pigs given 3.75 mmol/kg daily (1,152 mg/kg daily) in three divided doses, whereas in non-ascorbate deficient animals, toxicity was not seen at this dose, but were seen at double this dose.
Use of High-Dose Oral GSH in Cancer Patients
In one published study, eight patients with hepatocellular carcinoma were treated with 5 g oral reduced glutathione per day. Two patients withdrew shortly after receiving GSH due to intolerable side-effects (gastrointestinal irritation and sulfur odor). The remaining patients, aged 27-63, three male and three female, did not experience side-effects from this high dose of GSH and continued to take 5 g oral GSH for periods ranging from 119 days (at which time the patient died from her tumor) to >820 days (this patient was still alive at the time of publication and was still taking 5 g oral GSH daily; his tumor had not progressed and his general condition was good). Two of the female patients survived 1 year and exhibited regression or stagnation of their tumor growth. The remaining two patients, both male, died as expected within 6 months.
Experience in HIV-Infected Patients
A commercially available nutritional formulation containing 3 grams of reduced glutathione was given daily to a group of 46 AIDS patients for a period of three months by a group of private physicians. No significant GSH-related adverse effects were reported. No evidence of toxicities from laboratory studies or from clinical examinations was reported; however, no benefit was conclusively demonstrated.
Pharmacokinetics of Glutathione
The pharmacokinetics of intravenously administered GSH were determined in the rat and interpreted by means of an open, two-compartment model. Following a bolus injection of 50-300 mmol/kg GSH, arterial plasma concentrations of (i) GSH, (ii) oxidized glutathione/GSSG, (iii) total thiols, and (iv) soluble thiols minus GSH, were elevated and then rapidly decreased non-exponentially, as anticipated. With increasing dose, the rate constant for drug elimination and plasma clearance increased form 0.84 to 2.44 mL/min. and the half-life of the elimination phase decreased from 52.4 to 11.4 minutes. Both the apparent volume of distribution and the degree of penetration of GSH into the tissues were diminished with increasing dose (from 3.78 to 1.33 L/Kg and from 6.0 to 0.51 as k12/k21, respectively). The data indicate that GSH is rapidly eliminated. This is mainly due to rapid oxidation in plasma rather than by increased tissue extraction or volume distribution. Thus, plasma GSH levels appear to be quickly regulated by which the body may maintain concentrations within narrow physiological limits.
When single doses of 600 mg GSH were administered intravenously to sheep, GSH levels in venous plasma and lung lymph rose transiently. The mean concentration was approximately 50 mM for venous plasma, peaking at 30 min, and returning to baseline after 45 minutes. Lung lymph peak level was about 100 mM at 15 min, returning to baseline after 30 minutes. Average epithelial lining fluid (ELF) levels were variable but showed no significant increase over baseline during the three hour observation period. Urine excretion was rapid with peak levels at 15 minutes. In both plasma and lung lymph, GSH accounted for greater than 95% of the total glutathione (GSH plus GSSG). In ELF 75.4% of the baseline glutathione was in the reduced form, whereas in urine 59.6% was present as GSH.
Orally ingested reduced glutathione is absorbed intact from the small intestine in a rat model, specifically in the upper jejunum. It is noted that rat metabolism differs from man, and therefore the results of rat studies should be verified in man before the results are extrapolated. Plasma GSH concentration in rats increased from 15 to 30 mM after administration of GSH either as a liquid bolus (30 mM) or mixed (2.5-50 mg/g) in AIN-70 semi-synthetic diet (11). GSH concentration was maximal at 90-120 minutes after GSH administration and remained high for over 3 hours. Administration of the amino acid precursors of GSH had little or no effect on plasma GSH values, indicating that GSH catabolism and re-synthesis do not account for the increased GSH concentration seen. Inhibition of GSH synthesis and degradation by L-buthionine-[S,R]-sulfoximine (BSO) and acivicin showed that the increased plasma GSH came mostly from absorption of intact GSH instead of from its metabolism. Plasma protein-bound GSH also increased after GSH administration, with a time course similar to that observed for free plasma GSH. Thus, dietary GSH can be absorbed intact and results in a substantial increase in blood plasma GSH.
Administration of oral GSH increased hepatic glutathione levels in: (i) rats fasted 48 hours, (ii) mice treated with GSH depletors, and (iii) mice treated with paracetamol (a drug which promotes a depletion of hepatic GSH followed by hepatic centrilobular necrosis). In these experiments, the animals were orally intubated with 1000 mg/kg body weight GSH. Mean pretreatment values in 48-hour fasted rats were 3.0-3.1 mmol/g fresh hepatic tissue. Mean values after treatment were 5.8, 4.2, and 7.0 mmol/g fresh hepatic tissue for 2.5, 10, and 24 hours post-treatment, respectively. Mice were given an oral dose of GSH (100 mg/kg) and concentrations of GSH were measured at 30, 45 and 60 min in blood plasma and after 1 hr in liver, kidney, heart, lung, brain, small intestine and skin. GSH concentrations in plasma increased from 30 mM to 75 mM within 30 min of oral GSH administration, consistent with a rapid flux of GSH from the intestinal lumen to plasma. No increases over control values were obtained in most tissues except lung over the same time course. Mice pretreated with the GSH synthesis inhibitor BSO had substantially decreased tissue concentrations of GSH, and oral administration of GSH to these animals resulted in statistically-significant increases in the GSH concentrations of kidney, heart, lung, brain, small intestine and skin but not in liver.
The kinetics and the effect of glutathione on plasma and urine sulphydryls were studied in ten healthy human volunteers. Following the intravenous infusion of 2000 g/m2 of GSH the concentration of total glutathione in plasma increased from 17.5-13.4 mmol/Liter (mean =/−SD) to 823-326 mmol/Liter. The volume of distribution of exogenous glutathione was 176-107 Ml/Kg and the elimination rate constant was 0.063-0.027/minute, corresponding to a half-life of 14.1-9.2 minutes. Cysteine in plasma increased from 8.9-3.5 mmol/Liter to 114-45 mmol/Liter after the infusion. In spite of the increase in cysteine, the plasma concentration of total cyst(e)ine (i.e. cysteine, cystine, and mixed disulphides) decreased, suggesting an increased uptake of cysteine from plasma into cells. The urinary excretion of glutathione and of cyst(e)ine was increased 300-fold and 10-fold respectively, in the 90 minutes following the infusion.
Normal healthy volunteers were given an oral dose of GSH to determine whether dietary GSH could raise plasma GSH levels. Results showed that an oral dose of GSH (15 mg/kg) raised plasma glutathione levels in humans 1.5-10 fold over the basal concentration in four out of five subjects tested, with a mean value three times that of normal plasma GSH levels. Plasma GSH became maximal 1 hour after oral administration, dropping to approximately ½ maximal values after three hours. Equivalent amounts of GSH amino acid constituents failed to increase plasma levels of GSH. GSH bound to plasma proteins also increased with the same time course as seen with free GSH.